Microwave frequency heating apparatus



p 17, 1963 P. w. CRAPUC HETTES 3,104,303

MICROWAVE FREQUENCY HEATING APPARATUS Filed April 15, 1959 3Sheets-Sheet l Sept. 17, 1963 P. w. CRAPUCHETTES 3,104,303

MICROWAVE FREQUENCY HEATING APPARATUS Filed April 15, 1959 sSheets-Sheet 5 U UUTUTUUTPOOUFWUOO 06 United States Patent 3,104,303MICROWAVE FREQUENCY IEATING APPARATUS Paul W. Crapuchettes, Atherton,Calif, assignor, by

mesne assignments, to Litton Electron Tube Corporation, a corporation ofDelaware Filed Apr. 15, 1959, Set. No. 806,621 13 Claims. (Cl.219-41055) This invention relates to microwave frequency heatingapparatus, and more particularly to an improved microwave frequencyheating apparatus which employs a microwave oven which is excited in oneor more predetermined modes by at least one magnetron oscillator mountedflush within the walls thereof, and to the magnetrons therefor.

In existing ovens of the type which operate at microwave frequencies theheating thereof is achieved through the use of one or more highfrequency energy sources which communicate energy to the oven throughthe use of a transmission network such as coaxial line or waveguide. Inthe prior art it has been found preferable to use magnetrons almostexclusively as the source of energy for the illumination of the oven,the microwave energy being extracted from the magnetron usually by meansof an output coupling antenna which then forms the inner conductor ofthe output transmission line. More specifica'lly, the coupling antennausually consists of a coupling loop which has one end connected to thewall of the magnetrons resonant system, while the other end extends fromthe loop externally through the side wall of the magnetron along a pathperpendicular to the axis of the magnetron tube. The coaxial output lineis then used to either illuminate the oven directly through the use ofconventional coaxial probes inserted into the oven through openingsprovided in the walls thereof, or to excite a hollow waveguide connectedto the oven walls at some specific location where openings are providedfor physically terminating the waveguide.

While such prior art configurations will function to supply the requiredenergy to the associated oven cavity, there are nevertheless severalserious disadvantages attendant their use. Firstly, from an economicstandpoint the magnetrons generally envisaged for use with microwaveovens are of the type formerly associated almost exclusively withmilitary applications, and are therefore relatively expensive tofabricate. Moreover, the utilization of high power coaxial lines andwaveguide runs for introducing energy into the oven provides anadditional cost factor which must be considered and also introduceselectrical losses and reflections which effectively decrease the amountof power deliverably to the oven from any given tube. Secondly, from anoperational point of view these coaxial lines and Waveguide runs areknown to operate as a resonant load when connected between the magnetronand the oven cavity, thereby presenting mismatches or standing waves tothe magnetron which may cause it to oscillate unstably at a plurality ofoutput frequencies. As a consequence, the oven cavity is excited with aplurality of unpredictable mode patterns corresponding to the spuriousfrequencies at which the magnetron oscillates. Thirdly, the use ofadditional waveguide runs and transitions decreases substantially thevolumetric efiiciency of the overall oven while simultaneously adding tothe weight thereof.

In the prior art it has also been found through experience that certainundesirable difficulties arise when a system of two or more magnetronsoperating at substantially the same frequency are employed for excitingan oven, these difiiculties arising from the fact that when suchmagnetron devices each have substantially the same Patented Sept. 17,1963 output frequency, one of the devices tends to act as a lowimpedance across the output of the other due to slight dissimilaritiesbetween them, so that one of the magnetrons tends to absorb themicrowave power of the other, or stated difierently, acts as anadditional load across the other instead of supplying microwave power tothe common load of the cavity.

In the prior art, attempts have been made to avoid this latterlimitation by the use of magnetrons which produce output signals atappreciably different frequencies, the theory behind this concept beingthat the tendency of one of the magnetrons to absorb microwave powerfrom the other may be reduced to a level that is inappreciable. Althoughthis method has been used with minor success, it has proven impracticalfor many applications, and requires that two diiferent tube types bemanufactured and stocked to insure adequate servicing.

The present invention obviates the foregoing and other disadvantages ofthe prior art by providing a microwave oven which is excited by one ormore magnetrons mounted at preselected points in the oven walls, each ofthe magnetrons having an output window disposed substantially flush withan oven wall for launching energy directly into the oven. Moreparticularly, in accordance with one of the basic features of theinvention there is provided a magnetron having a resonant system whichexhibits substantially uniform impedance from tube to tube and whoseoutput energy waves are transmitted from the magnetron through anaxially positioned output window suitable for mounting in the oven wallsto directly excite the oven cavity. Moreover, as will be disclosed inmore detail hereinbelow, the advantages derived through the utilizationof an axially mounted output window are obtained through the use of arelatively simple magnetron structure which should provide an economicimpetus to the manufacture and sale of microwave ovens in general.

In accordance with another of the basic concepts of the invention, thereis provided a microwave oven which is excited directly by a pair ofmagnetrons whose inputs to the oven are positioned relative to eachother so as to illuminate the oven alternately in two different modeswithout appreciable intercoupling, thereby providing means for obtainingrelatively uniform energy distribution throughout the oven whilesimultaneously permitting the magnetrons to function at the same orrelatively close frequencies of operation.

It is, therefore, an object of the invention to provide an improvedmicrowave heating apparatus which employs direct excitation of amicrowave oven. Another object of the invention is to provide a metallicmicrowave oven which is excited by one or more magnetrons mounted withinthe walls of the oven to provide direct microwave illumination of theoven.

A further object of the invention is to provide a magnetron tube fromwhich energy is extracted through an output window in a directionparallel to the axis of the tube.

Still another object of the invention is to provide a magnetron whichemploys an output Window mounted coaxially with respect to the axis ofthe magnetron to provide direct coupling between the magnetron and anassociated microwave oven into a wall of which the magnetron is mounted.

A further object of the invention is to provide a microwave oven whereinenergy is coupled into the cavity alternately from a pair of magnetronsoperating at or near the same frequency and positioned to provideexcitation in two difierent modes for producing a relatively uniformpower density within the oven.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and wave frequencies.

advantages thereof, will be better understood from the followingdescription considered in connection with the accompanying drawings inwhich several embodiments of the invention are illustrated by way ofexample. It is Y to be expressly understood, how ever, that the drawingsare intended for the purpose of illustration and description only, andare not intended to limit the scope of the invention.

FIGURE 1 is a fragmentary view, partly in crosssection, of a microwaveoven illustrating the manner in which a pair of magnetron oscillatorsconstructed in accordance with the invention may be mounted within onewall of the oven to provide uniform excitation thereof;

FIGURE 2 is an enlarged cross-sectional view of one of the magnetronoscillators shown in FIGURE 1 illustrating the structural detailsthereof;

FIGURE 3 is a fragmentary view, partly in crosssection, of a microwaveoven which includes two associated magnetrons mounted within differentoven walls, in accordance with the present invention, to provide uniformexcitation thereof; and

' FIGURE 4 is a schematic diagram of one form of power supply unit whichmay be employed for exciting two magnetrons in accordance withtheteachings of the present invention.

' With reference now'to the drawings, wherein like or correspondingparts are designated by the same reference characters throughout theseveral views, there is shown in FIGURE I a microwave oven, generallydesignated 10, only a portion of which is shown, and a pair ofassociated magnetron oscillator tubes 12 and 13, constructed inaccordance with the teachings of the invention, which are mounteddirectly in the oven wall and energized alternately to provide microwaveenergy to heat any lossy dielectric body which may be placed within theoven.

14 having a pair of spaced apertures 15 passing through one wall 16 forpermitting the magnetron oscillators to be mounted therein, and anenclosure door 18to provide a communicating passage for placing objectswithin the oven or for removing objects previously placed therein.

It will be seen from FIGURE 1 that the oven is illuminated directly fromthe magnetrons 12 and 13 without the use of any additional microwavetransmission components.

' Considering how the specific elements of the microwave oven of theinvention, enclosure is preferably rectangular in form comprising sixmetallic walls. In the specific embodiment of FIGURE 1 enclosure door 18is located in one of the sidewalls, while the top wall 16 includesapertures 15, the configuration of the apertures depending upon theoutline of the magnetrons utilized therewith. In actual practice eachaperture has associated therewith a suitable grounding mechanism, suchas a plurality of spring fingers 20, to insure good electrical contactbetween the oven wall and the magnetron. will be seen, therefore, thatthe boundary Walls of the enclosure and enclosure door present to themagnetrons a cavity resonator which is capable of resonating in any oneof a plurality of; modes and at a plurality of microbelow, the oven maybe made to support oscillations in preferred mode configurations throughproper selection of the dimensions of the enclosure, the position ofeach magnetron in the oven wall, and the frequency of operation of themagnetrons. V,

Consider now the construction of the magnetrons mounted in the microwaveoven of FIGURE 1 and the manner in which they function to provide flushmounting capabilities. Referring now to FIGURE 2, there is shown adetailed view of a magnetron oscillator constructed in accordancewith'the teachings of the present invention,

As shown in FIGURE 1, oven 10 comprisestwo basic elements, namely, ametallic conductive enclosure anode to the region of the output window.

As will be disclosed in more detail oven along a path concentric withthe axis of the magand mechanically connected'to a pair of terminalmembers 37 and 38 which form part of cathode assembly 34. In addition,the magnetron includes a pair of pole pieces 42 and 4-4 which aredisposed on opposite ends of the anode structure and are spacedtherefrom by a pair of pole piece spacers 46 and 48 fabricated from anonmagnetic material, such as copper.

It should be noted at this point that the configuration of housingelement 26 and the placement of the various structural elements therein,in accordance with the invention, greatly facilitates construction andassembly of the magnetron. More specifically, it will be noted thathousing member 26 has a general cup shaped configuration with anaperture for receiving the cathode assembly. In assembling themagnetron, therefore, there are successively placed within the housing26 the pole piece 44, spacer 48, anode 35, spacer 46 and pole piece 42,thereby providing a sandwich type construction.

Considering now in more detail the construction of anode 35, it will benoted that the anode is basically a hybrid form of interdigitalstructure comprising a pair of cup-like conjugate members which whenplaced together form a cavity enclosed at its ends by a pair ofconductive end plates 54 and 56 positioned adjacent the opposite ends ofthe cathode 36 and perpendicular to the axis thereof. Each of the endplates has an aperture 55 in the central region thereof coaxial with thecathode, .and a group of anode fingers 57 mounted about this aperture atuniformly spaced points and extending perpendicular from the plane ofthe associated end plate toward the other end plate. cup-like anodemembers are positioned with respect to each other so that the two groupsof anode fingers respectively associated with the two end plates areinterleaved to form the anode inter-action structure of the interdigitalanode 35.

- Considering now the novel output structure employed in the magnetronof this invention, energy'is extracted from the magnetrons resonantsystem through a communicating passage 58 which extends through endplate 56, spacer 46, and pole piece 42 into the region 61 between polepiece 42 and outputwindow 30. More particularly, as viewed in FIGURE 2,the communicating passage is formed by an aperture eccentrically locatedin each of the foregoing elements, the apertures being drilled orotherwise formed to register with each other and thereby provide apassage substantially parallel with the tube axis, which extends fromthe inductive region of the magnetron It should be noted at this point,'zthatif desired, energy may be extracted directly from the magnetronthrough passage 58 "by using the passage. as a cylindrical wave guideinto which the output energy propagates. However, in accordance with thepreferred embodiment of the invention the output structure furthercomprises a conductor 60 which extends from the anode resonator, throughpassage 58 and into the region 61 adjacent output window 30 whereat itis terminated at a point 72 on the antenna in the inductive region ofthecavity resonator,

the output energy thus produced'being transmitted to a the outputantenna by the coaxial link formed by passage As further shown in FIGURE2, the two be understood, of course, that if desired the central regionof conductor 60 may be further supported within passage 58 through theuse of a suitable insulating spacer, such as the spacer designated 64 inFIGURE 2.

In the operation of the magnetron the RF electric field set up in theresonant system functions to generate a current in conductor 60, andenergy is extracted through the coaxial link formed by passage 58 and ispresented to the output antenna portion of conductor 60 in the region61. At this point, therefore, the conductor 69 functions in conjunctionwith the reflective back surface of pole piece 42 to produre a radiatedfield in response to the current flowing in the conductor. It should benoted, incidentally, that the process by which microwave oscillationsare generated in the magnetron of the present device is substantiallythe same as that of conventional magnetrons, and therefore furtherdiscussions of the phenomena occurring in the electron inter-action gapis considered unnecessary.

Consider now the advantages of the construction of the magnetron ofFIGURE 2 with respect to its use in magnetron ovens and the manner inwhich it provides flush mounting capabilities. In accordance with theinvention the microwave energy generated within each magnetron isoriented uniquely by the antenna-like conductor 60 in such a manner thatit may be launched in an output wave through the output window along apath which is coaxial with the axis of the magnetron. This novel featureof orientating the output energy wave differs substantially from that ofthe prior art devices and is utilized advantageously as describedherein, to permit the magnetron to be mounted in the walls of the oven,thus elimimating the need for conventional lossy and ineflicienttransmission circuits and microwave plumbing traditionally associatedwith the microwave ovens of the prior art. A further advantage isderived from the fact that the novel construction of the magnetronoutput structure functions topermit the magnetron to be placed readilyin any one of various locations within the oven Walls to producewhatever mode pattern has been selected for the operation of the oven.

Returning now to the description of FIGURE 1, the static magnetic fieldrequisite for magnetron operation is provided in part by a pair ofannular permanent magnets 80 and 82 respectively associated withmagnetrons 12 and 13, and in part by a pair of electromagnets comprisingtwo electromagnet coils 84 and 86 and associated magnetic returnstructures. It will be noted that each of the magnetic return structuresincludes an annular member 88 which engages the associated permanentmagnet, and a yoke 96 which communicates magnetically with the oppositeends of the evacuated envelope of the associated magnetron, the yokes 90further providing a surface for fastening the magnetrons in positionwith a suitable clamp 92.

It will be further noted from FIG. 1 that in addition to the elementsdescribed previously, each of the magnetrons further comprises aplurality of annular cooling fins 93 which are preferably formed of agood heat condoctor, such as copper, the fins being stacked together andaffixed to the tubular body portion 26 of each of the magnetrons. Whilean air cooled system of this nature will be found satisfactory in mostapplications, it will of course be appreciated by those skilled in theart that an equivalent liquid cooling system could be employed with thebasic magnetron construction shown in FIG. 2 without departing from thebasic concept of the invention.

Consider now the factors which determine the position of the magnetronsin the oven wall 16 as shown in FIG- URE 1. It will be recognized bythose familiar with resonator design that the dimensions of the oven andthe frequency of operation of the magnetrons both contribute toestablish the number of mode pattern-s of the type TE which may beexcited in the oven, and that the excitation of a preselected modeWithin the oven by a particular magnetron is also dependent upon theexcitation energy therefrom being introduced into the oven at a givenpoint such that it has a particular space relationship to the fields ofthe mode pattern to be excited. Furthermore, it may be demonstrated thatto achieve a relatively uniform power density throughout the oven it ispreferable to excite the oven alternatively in two different modes whosefield configurations overlap, and that to minimize any coupling betweenthe two magnetrons each of the tubes should be located preferably at apoint in the oven wall which appears substantially as a null in the modepattern produced by the other tube.

In general the frequency range in which frequency of operation of themagnetrons must fall is preordained by the Federal CommunicationsCommission. Accordingly, after the specific frequency at or around whichthe magnetrons will operate has been selected, it is only necessary toselect two mode configurations for exciting the oven which areconsistent with the criteria outlined above and which provide overalloven dimensions compatible with the size of conventional ovens for usein environments similar to that for which the microwave oven isintended. In additon, once the mode patterns of the type TE which are tobe utilized in the oven are selected, the locations for placement of themagnetrons can be determined, and the magnetrons placed in spacedrelationship to the fields of the mode to provide proper excitation ofthe oven. Since there are many possible modes in which the oven mayoperate for a given frequency, it is important that placement of themagnetrons be relatively accurate to insure proper coupling to the ovenso that excitation occurs in the particular modes selected to provideuniform heating Within the oven. Thus, as an added feature of theinvention, the use of flush mounted magnetrons as shown in FIG. 1facilitates placement of the magnetrons for coupling energy into theoven directly at the desired points through the use of the apertures 15described hereinabove.

It is to be expressly understood, of course, that the invention may bepracticed in manners other than that shown in the embodiment of FIGURE 1by employing a plurality of magnetrons of the general form shown inFIGURE 2. With reference now to FIGURE 3, for example, there is shown amicrowave cooker in accordance with the invention which includes an oven10 and two magnetrons I12- and 13 for the illumination thereof, themagnetrons being positioned in adjacent walls of the oven. In thisembodiment of the invention the magnetrons function in substantially thesame manner and provide the same advantages as in the embodiment ofFIGURE 1, one further advantage of the invention illustrated in FIGURE 3being that the magnetrons may be placed in the various walls of the ovenfor special applications.

It will be recognized, of course, that numerous electrical circuits maybe devised for energizing magnetrons 12 and 13 alternatively in time.With reference to FIG- URE 4, for example, there is shown a magnetronpower supply which comprises a power transformer 94 having acenter-tapped secondary which is connected to the anodes of magnetrons12 and -13 through a pair of respectively associated electromagnetwindings 84 and 86. The cathodes of the magnetrons, in turn, are eachconnected to the ends of the transformer secondary by a pair ofasosciated conductors, the ends of the secondary further providingfilament transformers for energizing the magnetron cathodes. Inaddition, as shown in FIGURE 4, the circuit further includes a pair ofswitch contacts 96 and 98 which are selectively operable in unison toshort circuit a portion of electromagnet coils 84 and 86 to therebyprovide means for varying the output power presented by the magnetronsthrough variation of the magnetic field applied to the associated tubes.It will be recognized by those skilled in the art that maximum powerwill be presented when the switches are in the small coil position sincethe transverse magnetic field in the tubes is then held to a specifiedminimum whereas a smaller amount of power will be presented when thefull transverse magnetic field is applied to the tubes.

[It is to be understood, of course,that alterations and modificationsmay be made in the structure and circuits shown without departing fromthe spirit and scope of the invention. closed herein are equallyapplicable for use with a pair of magnetrons placed in opposite walls ofthe oven, for operation of the magnetrons at substantially differentfrequencies, or for the use of more than two magnetrons as an energysource. It should be emphasized, therefore, that the spirit and scope ofthe invention is to be limited only by the spirit and scope of theappended claims.

What is claimed as new is:

l. The combination of a microwave oven and a plural. ity of magnetronoscillators to drive said oven in a plurality of respective modes eachhaving at least one null point, said combination comprising: a microwaveoven having a plurality of metallic walls defining acavity resonator; afirst magnetron oscillator for illuminating said oven with microwaveenergy, said magnetron including a substantially cylindrical housingmember having an axis and first and second ends, a cathode, meansenclosing said first end of said housing member for mounting saidcathode in said member concentric with said axis, an anode memberdefining a resonator system concentrically surrounding said cathode,said resonator system including at least one cavity resonator meansincluding an output Window transparent to microwave energy enclosingsaid second end of said housing member, and means for extractingmicrowave energy from said anode member and launching said energy in anoutput wave through said output window; and means positioned at a nullpoint of a resonant mode of a second magnetron oscillator for mountingthe lfirst magnetron in one of said walls of the oven with said outputwindow substantially flush with said one wall whereby said oven isexcited in a mode of preselected orientation relative to the resonantmode of said second magnetron oscillator.

2. The combination defined in claim 1 which further includes a secondmagnetron oscillator having an output window, and second meanspositioned along one of said metallic walls at a null point of saidresonant mode of said first magnetron oscillator for mounting saidsecond 'magnetron therein whereby said oven is excited in a second modeof preselected orientation.

*3. The combination defined in claim 2 wherein said magnetronoscillators are positioned relative to each other and to said oven wallsto energize said oven in two different modes, and which further includesmeans for energizing said magnetron oscillators alternately in time. v

4. The combination comprising: a metallic enclosure whose interiorboundary walls define a cavity resonator resonant at microwavefrequencies and in a plurality of modes, in which a lossy dielectricbody may be placed to be heated, a magnetron for generating microwaveenergy,

said magnetron having an axis and including an axially mounted cathode,an anode structure positioned concentrically around said cathode, saidanode defining at least one cavity resonator, means including asubstantially fla-t output window coaxial with said cathode torextracting the microwave energy from said anode resonator, and means formounting said fiat magnetron output window with its exposed surfacesubstantially flush with one wall of said cavity resonator in apreselected location to provide direct coupling of the microwave energyinto the interior of said metallic enclosure and to excite a preselectedmode therein.

5. The combination comprising: a microwave oven hav ing a plurality ofboundary walls defining a cavity resonator resonant in a plurality ofmodes; a plurality of magnetron oscillators each having an interdigitalanode structure for generating microwave energy, each oscillator Forexample, it is clear that the concepts dismode from each of saidoscillators; means for energizing said oscillators alternately; andmeans for mounting said oscillators flush within the oven walls at twodifierent places for coupling the microw-ave energy of said oscillatorsdirectly into the interior of the oven to excite the same in at leasttwo of said modes and to prevent s-ubstantially the exchange of energybetween said magnetron oscillators, said oscillators transmittingmicrowave energy to the interior of said area alternately duringdifferent periods to provide uniform average energy at substantially 7all points within the oven, such that said microwave oven netron'oscillators each having an interdigital anode structure for generatingmicrowave energy, each oscillator cooperating with said cavity toprovide a difierent output mode from each of said oscillators; means forenergizing said oscillators alternately; and means ior mounting saidoscillators flush within the oven walls at two different places :forcoupling the microwave energy of said oscillators directly into theinterior of the oven to excite the same in at least two of said modesand to prevent substantially the exchange of energy between saidmagnetron oscillators, said oscillators transmitting microwave energy tothe interior of said area alternately during different periods toprovide uniform average energy at substantially all points Within theoven, the dimensions of said oven, the position of said'magnetrons insaid oven walls, and the frequency of operation of said magnetronsbeingchosen v to provide excitation of said oven in the TE mode by oneof said magnetrons and in the TE mode by'the other of said magnetro-ns,Where the terms I, l, w, w, h and h are integers and at least two ofsaid terms I, w and h have different integer values than thecorresponding two prime terms.

7. In a magnetron oscillator, the combination comprising: asubstantially cylindrical housing member having an axis; a cathode;mounted in said member concentric with said axis; an anodemember mountedwithin said housing member concentric with said cathode, said anodemember defining at least one cavity resonator; means including an outputwindow transparent to microwave energy enclosing one end of said housingmember; and means in cluding a trans-mission iris and a continuouspassageextending from within said cavity resonator and parallel to saidaxis for extracting microwave energy from said anode member and axiallylaunching said energy in an output wave through said output window. 7

8. In a magnetron the combination comprising: a cylindrical cathodestructure, an anode defining a resonant cathode and forming a section ofa continuous transmission line passage extending parallel to saidmagnetic field and longitudinally of said cathode structure 'from withinsaid resonant cavity to provide a communicating opening for extractingmicrowave energy from said resonant cavity and longitudinallygof saidcathode structure.

. 9. A magnetron comprising: a vacuum envelope, said envelope includinga cylindrical housing member having first and second ends, an outputwindow electrically transparent to microwave energyhermeticallyenclosing said first endof said housing member, and a cathode terminalassembly hermetically enclosing said second endof said housing member; acathode mounted axially within said cylindrical housing member by saidterminal assembly; a

cylindrical anode structure defining a resonant cavity for generatingmicrowave energy, said anode having an inductive portion and first andsecond ends parallel to said output window and being positioned withinsaid housing member to concentrically surround said cathode forproviding an interaction space between said cathode and anode; a pair ofmagnetic pole-pieces one adjacent each end of said anode structure andcoaxial with said cathode, one of said pole-pieces separating said anodestructure and said window and being spaced from said window, said onepole-piece having an aperture therethrough extending between said anodestructure and the volume between said window and said one pole piece to[form a continuous transmission passage extending longitudinally of saidcathode and from within said resonant cavity for extracting energy fromsaid anode structure and longitudinally launching said energy throughsaid output window.

10. The magnetron defined in claim 9' which further includes a conductorextending from said inductive portion of said anode resonant cavity,through said aperture in said one pole-piece, and into said volumebetween said one pole-piece and said window, said conductor functioningas an inductive receiving antenna for extracting energy from within saidanode cavity and as a transmitting antenna for radiating the extractedenergy through said output window.

'11. A magnetron comprising: a vacuum envelope including a cylindricalhousing member having first and second ends, an output windowelectrically transparent to microwave energy hermetically enclosing saidfirst end of said housing; a cathode mounted axially within saidcylindrical housing member; an interdigital anode structureconcentrically surrounding said cathode and coaxial therewith forgenerating microwave energy within said magnetron, said anode structureincluding a pair of conductive end plates mounted within said housingmember adjacent the opposite ends of said cathode and perpendicular tothe axis thereof, each of said end plates having a first aperture in thecentral region thereof, coaxial with said cathode a first group of anodefingers mounted at uniformly spaced points to one of said platesadjacent said first aperture and extending toward the other of saidplates, parallel with said cathode a second group of anode fingers,said'second group of anode fingers being mounted at uniformly spacedpoints to said other plate adjacent said first aperture and extendingtoward said plate parallel with said cathode, said first and secondgroups of fingers being interleaved to provide an interdigital anodestructure, said interdigital anode structure forming a radialtransmission line having a capacitive port-ion adjacent said cathode andan inductive portion remote from said cathode, the end plate nearest tosaid output window having a second aper- 1 0 ture passing therethrougheccentric with respect to said cathode to provide inductive couplingmeans for extracting microwave energy generated within said anodestructure; a pair of magnetic pole-pieces one adjacent each end of saidanode structure and coaxial with said cathode, one of said pole-piecesseparating said anode structure and said window and being spaced fromsaid window, said one polepiece having an aperture therethroughextending between said anode structure and the volume between saidwindow and said one pole-piece for extracting energy from said anodestructure and launching said energy through said window.

12. The magnetron defined in claim 11, which further includes aconductor extending from the inductive portion of said interdigitalanode structure, through said second aperture in said end plate and saidaperture in said one pole-piece, and into said volume between said onepolepiece and said window, said conductor functioning as a receivingantenna for extracting energy from said anode structure and as atransmitting antenna for radiating the extracted energy through saidoutput window.

13. The combination comprising: a metallic enclosure whose interiorboundary walls define a cavity resonator resonant at microwavefrequencies and in a plurality of modes, in which a lossy dielectricbody may be placed to be heated, a magnetron for generating microwaveenergy, said magnetron having an axis and including an axially mountedcathode, an anode structure positioned concentrically around saidcathode, said anode defining at least one cavity resonator, meansincluding a substantially fiat output window for extracting themicrowave en ergy from said anode resonator, and means for mounting saidflat magnetron output Window with its exposed surface substantiallyflush with one wall of said cavity resonator in a preselected locationto provide direct coupling of the microwave energy into the interior ofsaid metallic enclosure and to excite a preselected mode therein.

References Cited in the file of this patent UNITED STATES PATENTS

5. THE COMBINATION COMPRISING: A MICROWAVE OVEN HAVING A PLURALITY OFBOUNDARY WALLS DEFINING A CAVITY RESONATOR RESONANT IN A PLURALITY OFMODES; A PLURALITY OF MAGNETRON OSCILLATORS EACH HAVING AN INTERDIGITALANODE STRUCTURE FOR GENERATING MICROWAVE ENERGY, EACH OSCILLATORCOOPERATING WITH SAID CAVITY TO PROVIDE A DIFFERENT OUTPUT MODE FROMEACH OF SAID OSCILLATORS; MEANS FOR ENERGIZING SAID OSCILLATORSALTERNATELY; AND MEANS FOR MOUNTING SAID OSCILLATORS FLUSH WITHIN THEOVEN WALLS AT TWO DIFFERENT PLACES FOR COUPLING THE MICROWAVE ENERGY OFSAID OSCILLATORS DIRECTLY INTO THE INTERIOR OF THE OVEN TO EXCITE THESAME IN AT LEAST TWO OF SAID MODES AND TO PREVENT SUBSTANTIALLY THEEXCHANGE OF ENERGY BETWEEN SAID MAGNETRON OSCILLATORS, SAID OSCILLATORSTRANSMITTING MICROWAVE ENERGY TO THE INTERIOR OF SAID AREA ALTERNATELYDURING DIFFERENT PERIODS TO PROVIDE UNIFORM AVERAGE ENERGY ATSUBSTANTIALLY ALL POINTS WITHIN THE OVEN, SUCH THAT SAID MICROWAVE OVENIS EXCITED IN AT LEAST TWO DIFFERENT MODES OF THE FORM TELWH DURING SAIDDIFFERENT PERIODS, WHERE L, W AND H ARE INTEGERS OF PREDETERMINEDVALUES.